Racing to find survivors buried in the rubble of collapsed buildings before it's too late, specialized search and rescue teams know their only indication that a victim is trapped may be a faint scratching or a weakened cry for help.

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It's a difficult task in a densely populated city like Port-au-Prince, where the city's approximately 3 million residents have flooded the crumbled streets afraid to seek shelter because of continuing aftershocks.

Yet when the calls go out, the silence is largely granted even by those who are the most panicked.

"They know the rescuers are using this to save people," said Battalion Chief Bob Zoldos of the Fairfax County Fire and Rescue unit, one of the first to reach Haiti.

And it's not as simple as hushing bystanders. Simple background noise such as the sound of running water or rescue crews working to free a victim five buildings away can throw off a team's search, wasting valuable time.

While on his first mission to the 1998 embassy boming in Kenya, "we would actually announce to everyone there that we need silence and the amazing thing was that the chaos actually did stop," Zoldos said.

The next year, when he was in Turkey after the massive earthquake that killed more than 18,000 people, victims did the same.

"That was more difficult, but still you could get more people in the area to quiet down," he said. "It can be done."

And in a place like Haiti, where citizens are desperate to find their loved ones, Zoldos said he expects similar cooperation.

Zoldos' colleagues with the Virginia Task Force 1 are already at work in Haiti -- 72 rescuers were dispatched Wednesday along with six dogs. Zoldos will lead the second team being prepped for deployment.

Silence Among the Chaos: Finding Earthquake Victims

The difficulty with finding survivors trapped in what's left of the buildings is knowing whether anyone is even in there. Tapping noises could be someone's cry for help or the sounds of a weakened building settling.

The dogs are brought in immediately, Zoldos said, because their ability to sniff out a survivor and detect low level sounds are far superior to anything their human team members could do.

As their handlers listen for the dog's bark, teams pull out seismic listening devices that will give rescuers rough coordinates of where the survivor's are in the rubble.

Roughly the size of a soda can, the orange devices are reliable at picking up vibration, Zoldos said, but can easily be fooled by outside noise.

"When you're looking for scratching or something like that or someone tapping it is very hard to distinguish," he said.

Now nearly 48 hours into the recovery effort, Zoldos said, rescuers are eager to get those people out.

"There's been rescues all the way to day nine, 10 and 11 depending on the injury of the person, the environment, the exposure -- all that plays into their suvivablility," he said. "If there's any chances people are there, we want to search it."

Right now crews in Haiti are in "blitz mode," working until they can't physically go any longer. In Turkey, he said, his team went 56 hours without stopping.

Out in California, fire captain Marc Valentine boarded a bus this morning to start his trip to Haiti. Accompanied by Val, his 12-year-old golden retreiver/laborador mix and partner on the California Task Force 5 out of Orange County Fire and Rescue, Valentine said just the small amount of time he's spent trapped during training excercises make him eager to get down there.

"When I got my dog from Search Dog Foundation and started burying ourselves in the piles, I got a true empathy for the victim," he told ABCNews.com.

"We're coming up on 48 hours these people have been buried," he said. "And how uncomfortable it is to be buried in for an hour -- I can only imaging the terror they are feeling buried in those holes."

Valentine and Val -- who will be retiring after his tour of duty in Haiti -- are expected to land in Haiti tonight.

"You look at huge buildings. We're looking at an island the size of Rhode Island -- where do you start? he said. "These dogs give you the best opportunity to find that starting point."

Finding Haiti's Victims: 'He's Going to Find Them'

While many watching disaster coverage see the dogs as having a heroic drive to find the suffering and please their masters, Val and the other dogs with him will be working for one reason only -- to play tug afterward.

"The dogs do it for one reason only. They think the person they find will play tug toy with them," Janet Reineck, director of development for Search Dog Foundation, told ABCNews.com. "Because they've been trained day in and day out that whenever they find a person they can't see, they pull on a tug toy with them."

Search Dog Foundation has six dogs in Haiti and another nine, including Val, on the way. And they are just as useful, if not more so, then the seismic listening devices or tiny cameras the rescue teams employ.

"They are trained very, very heavily to ignore all distraction – sounds, smells, food," she said. "They have to be trained to ignore anything but the scent of a person they cannot see."

And yet the calls for quiet needed by their human counterparts are also necessary to let the dogs work. The dogs train for months to work off leash and when a team has got one dog racing around the harsh terrain of a collapsed building, they need to be able to hear that dog's bark from wherever he comes to a stop.

"The bark alert is what it's all about," Reineck said.

After the Sept. 11, 2001 attacks, the mounds of twisted steel and dusty concrete would be teeming with emergency workers until they heard the call, "Clear the rubble, the dog is on!" Quickly scrambling off the wreckage, the teams would stand silently as the dog raced across the rubble trying to find anyone still alive.

And if search and rescue dogs clear building after building without finding any survivors, their handlers are trained to bury themselves secretly to give their dogs something to find.

"There's this notion they get depressed. They don't get depressed," Valentine said. "They get bored."

Valentine's team member Val was initially trained to be a service dog for the handicapped, but flunked out after his high energy drive became unworkable.

But that drive is exactly what Valentine wants by his side in the next two weeks.

"This is the real game," he said. "He's going to get out there and he's going to find them."